The present invention relates to a process for the removal of organic substances and nitrogen compounds from waste water by means of micro-organisms, in which the waste water to be treated is passed continuously through a denitrification zone and then through a nitrification zone, part of the effluent from the nitrification zone is recycled to the denitrification zone, and the remainder is discharged.
A process of this type has been described in British Patent Specification No. 1,438,697. According to this patent specification it is known that domestic waste water can be freed from ammonium nitrogen by oxidation of this ammonium nitrogen to nitrate nitrogen by means of nitrifying microorganisms in biologically active sludge with molecular oxygen and subsequent reduction of the nitrate to molecular nitrogen by means of denitrifying microorganisms. Any nitrate nitrogen and nitrite nitrogen contained in the waste water will be removed at the same time.
Ammonia, ammonium ions and compounds that yield ammonia and ammonium ions in the course of the process are referred to as `ammonium nitrogen` in this application, and nitrate ions and nitrite ions are referred to as `nitrate nitrogen`.
The second stage of this known process, the denitrification, requires the use of an oxygen acceptor, which may be, e.g., an oxidizable organic substance, such as methanol. A useful oxidizable organic substance is usually present in the waste water to be treated, but only a small part of it can reach the second stage of the purifying process, as the organic substance has been reduced to a large extent in the nitrification stage by biological oxidation with atmospheric oxygen.
The British Patent Specification No. 1,438,697 proposes to reverse the order of treatment, and to subject the waste water first to a denitrification and then to a nitrification. The main advantage of this procedure is that the oxygen acceptor required for the denitrification is supplied by the oxidizable organic substances contained in the waste water to be treated. As the effluent from the nitrification zone is not free of nitrate, over 50% of the effluent is recycled to the denitrification zone to reduce the nitrate content of the liquid discharged.
In the denitrification zone, which is the first zone in the known process, nitrate nitrogen is reduced to molecular nitrogen and the organic impurities are oxidized.
This oxidation of the organic substances fed in needs a given amount of nitrate. When a comparatively small amount of nitrogen compounds are contained in the waste water, as is the case in domestic waste water, the supply of nitrate to the denitrification zone will be insufficient for the oxidation of the organic impurities fed in. This gives rise to processes, such as accumulation of organic material in the sludge causing less desirable growth of bacteria, and other undesirable anaerobic processes that may affect the sludge quality, notably the settling properties. This implies that the concentration of suspended sludge in the system is low and that a large reactor is required. An additional consequence is that the after-settlement must have considerably larger dimensions because the settling rate is lower, while this sludge may cause problems when subjected to further processing, such as drying.